The objective of this research is to begin the study of the primary amino acid sequence of the serum thyroid transport protein, thyroxine binding globulin (TBG). We propose to carry out the initial cleavages of purified TBG at methionine residues with cyanogen bromide and at arginine residues with trypsin. We next intend to isolate the peptides generated by these cleavages by means of gel filtration and ion exchange column chromatography. When pure peptides have been obtained, their amino sequence will be obtained by automated Edman degradation and the Pth amino acid derivatives will be identified by high performance liquid chromatography. It is hoped that some amino acid sequence overlaps of the peptides obtained from these cleavages will be found and a partial amino acid sequence of TBG can be elucidated. The work proposed is essential for further studies on the complete amino acid sequence of TBG.